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Examples of Tempering.


heat from the body b c travels down towards a, the colours will
appear, the point becoming gradually hotter, yellow first, then
through brown to blue. But we require for our chisel the tem-
perature of 550°, which is indicated by a dark purple; as soon,
then, as this tint is seen, the chisel is entirely plunged into water,
and the point is thus made of the correct degree of hardness. A
drill point may be tempered in a similar manner, using, however,,
the darker straw yellow for colour.

^ Tempering a Screw-tap.—Sometimes, when an even
temper is required over a considerable surface, the result may be
better obtained by putting the article in contact with a body oi
hot metal. Such a case is that of a screw-tap. The tool, being
finished and polished, is next to be so tempered as to make the

screw threads hard, while the square shank remains soft. An
iron tube being procured, A, Fig. 123, of such a diameter as to
just fit freely over the tap, the latter is first lifted by the shank,
within red-hot tongs, B. In the meantime the tube has been
heated to a dull red, and as soon as the first signs of straw colour
are seen on the tap shank, due to the heat from the tongs, the
tap is placed within the tube as shown. The .shank will have had,
so to speak, a start of the rest of the tap, and by tlie time c has
assumed a dark strawy colour, r> will have arrived at dark blue, a